Mangrove ecosystems are vital blue carbon reservoirs that store biomass and carbon while supporting coastal protection, biodiversity, and local livelihoods. In Timor-Leste, empirical data on mangrove carbon stocks remain limited, creating a major gap for conservation planning and climate policy. This study provides the first site-specific baseline of mangrove biomass and carbon storage in Sabuli Village, Metinaro, and evaluates the reliability of diameter at breast height (DBH) as a predictor for carbon monitoring. Field surveys were conducted using stratified random sampling, with tree diameters measured at 1.3 m height. Biomass was estimated through standardized allometric models and partitioned into above-ground biomass (AGB) and below-ground biomass (BGB). Correlation and regression analyses were applied to test the predictive capacity of DBH for biomass carbon parameters. Results showed that Sabuli’s mangroves contained an average of 48 t ha-1 of total biomass, comprising 35 t ha-1 AGB (72.9%) and 13 t ha-1 BGB (27.1%). These values corresponded to 23 t C ha-1, equivalent to 84 t CO2e ha-1. DBH exhibited very strong correlations with all biomass–carbon pools (r > 0.96; R2 > 0.92), with the strongest regression observed between DBH and BGB (R2 = 0.94). The findings highlight DBH as a reliable, non-destructive, and cost-effective indicator for mangrove carbon estimation. While carbon values in Sabuli are modest compared to pristine sites, they provide an essential baseline for Timor-Leste, supporting conservation strategies, national climate commitments, and potential blue carbon initiatives.